A 4-0 result seem to me as an exaggeration for this match. 
Milan will feel especially Ibrahimovic's absence in this match but I don't think they will lose this match with too many goal difference. I feel confident about Milan will also score but Man Utd will defeat them 2-1 or 3-1, in my opinion.
Manchester United are now stronger than Milan, all the more so in a home match they have to defeat their rival. I think Milan will play in this match with the expectation of a draw and will try to score a goal on the opponent's field. I'm not sure if they can do it. Milan will feel especially Ibrahimovic's absence in this match but I don't think they will lose this match with too many goal difference. I feel confident about Milan will also score but Man Utd will defeat them 2-1 or 3-1, in my opinion.